1. Field of the Invention
The present invention relates to a planar light source and a liquid crystal display using this planar light source. More specifically, the present invention relates to a planar light source capable of controlling brightness and a liquid crystal display using this planar light source.
2. Description of the Background Art
In many cases, a planar light source is intended for use as a backlight of a transparent liquid crystal display. A cold cathode fluorescent lamp (CCFL) is generally used as a light source part of this planar light source. An inverter is indispensably required to light the cold cathode fluorescent lamp.
An inverter is used to control the brightness of the planar light source. However, due to limitations of the characteristics of the cold cathode fluorescent lamp, only about 30 percent of maximum brightness can be obtained by the inverter alone. A liquid crystal display intended for use in a shipboard or in a car-mounted system is required to provide a wide range of brightness control in order to ensure safety in the nighttime.
As a result of the use in various environments, a liquid crystal display has been used in various temperatures. The cold cathode fluorescent lamp is unlikely to light up at a low temperature and in a low-current condition. The cold cathode fluorescent lamp, even when it lights up in these conditions, is subjected to the bias of encapsulated mercury, and hence is difficult to light up in a usual mode. Thus the planar light source suffers from a limited range of brightness control due to a low temperature.
In response, according to the technique introduced in Patent Application Laid-Open No. 6-222363 (1994), a light amount control member is interposed between a light source and a light guide plate to broaden the range of brightness control. More specifically, light emitted from the light source is absorbed by the light amount control member. Then the amount of light entering the light guide plate is controlled to thereby broaden the range of brightness control.
When the light amount control member is arranged between the light source and the light guide plate as taught in Patent Application Laid-Open No. 6-222363 (1994), the range of brightness control is broadened on the low brightness side while lowering maximum brightness. Namely, the light amount control member absorbs not a little amount of light even when the light amount control member is formed by a material with high transmittance. As a result, the existence of the light amount control member lowers maximum brightness.